The present invention relates generally to methods for dispensing flowable substances on microelectronic substrates, for example, methods for controlling a flow of a liquid photoresist onto a semiconductor wafer. Microelectronic features are typically formed in semiconductor wafers by selectively removing material from the wafer and filling in the resulting openings with insulative, semiconductive, or conductive materials. One typical process includes depositing a layer of light-sensitive photoresist material on the wafer, then covering the photoresist layer with a patterned mask, and then exposing the masked photoresist to a selected radiation. The mask is then removed and the entire photoresist layer is exposed to a solvent. In one case, the portions of the photoresist layer exposed to the radiation through patterned openings in the mask become resistant to the solvent. Alternatively, the portions covered by the mask become resistant to the solvent. In either case, the portions of the photoresist layer remaining on the wafer after being exposed to the solvent can protect the underlying structure when the wafer is subsequently exposed to an etchant. The etchant then creates a pattern of openings (such as grooves, channels, or holes) in the wafer material or in materials deposited on the wafer. These openings can be filled with insulative, conductive, or semiconductive materials to build layers of microelectronic features on the wafer.
One conventional arrangement for depositing photoresist on a semiconductor wafer is shown in FIG. 1A. An apparatus 10 (such as a DNS SK2000, available from Dai Nippon Screen of Kyoto, Japan) includes a substrate support 11 that supports a wafer 12. A dispense nozzle 43 is positioned above the wafer 12 to dispense a liquid photoresist 33 on a central portion of the wafer 12. The wafer 12 spins (as indicated by arrow xe2x80x9cAxe2x80x9d) to distribute the photoresist 33 over the upward facing surface of the wafer 12.
The apparatus 10 also includes a delivery system 40 that provides a regulated quantity of liquid photoresist to the dispense nozzle 43. The delivery system 40 includes a resist reservoir 41 coupled to a pump 42 to propel the photoresist to the dispense nozzle 43. A valve assembly 30 between the reservoir 41 and the dispense nozzle 43 regulates the flow of the photoresist to the dispense nozzle 43. The valve assembly 30 includes a dispense valve 31 that opens to allow the photoresist to flow to the dispense nozzle 43 and closes to prevent the flow of the photoresist. The valve assembly 30 further includes a suckback valve 32 that withdraws at least some of the liquid photoresist from the dispense nozzle 43 when the dispense valve 31 is closed, thereby reducing the likelihood for extraneous drops of photoresist to drip from the dispense nozzle 43. For example, as shown in FIG. 1B, the suckback valve 32 can operate to keep the photoresist 33 flush with the end of the dispense nozzle 43 or, (as shown in FIG. 1C) recessed from the end of the dispense nozzle 43 after the dispense valve 31 is closed. In either case, the suckback valve 32 is configured to prevent the photoresist 33 from extending beyond the end of the dispense nozzle 43 (as shown in FIG. 1D) when the dispense valve 31 is closed.
The dispense valve 31 and the suckback valve 32 are operated by air from a pressurized air supply 44. The flow of pressurized air to the valves 31 and 32 is controlled by electrically-operated solenoids 45a and 45b, respectively.
A computer-based controller 20 controls the operation of the solenoids 45a and 45b, and also controls the spin motion of the substrate support 11. Accordingly, the controller 20 includes a valve controller 23 operatively coupled to the solenoids 45a and 45b, and a spin speed controller 22 operatively coupled to a motor that rotates the substrate support 11.
The apparatus 10 can further include a video camera 21 operatively coupled to the spin speed controller 22. In operation, the video camera 21 can detect when a certain portion of the wafer 12 is covered with the photoresist 33. The speed controller 22 can then alter the speed with which the substrate support 11 spins, based on the image received from the video camera 21, to control the coverage of the photoresist 33 over the surface of the wafer 12.
One drawback with the conventional arrangement shown in FIG. 1A is that it can be difficult to accurately control the amount of photoresist 33 dispensed on the wafer 12. For example, dispensing even one additional drop of photoresist on a wafer can dramatically increase the amount of photoresist required to process a large number of wafers. Conversely, dispensing too little photoresist on the wafer can produce an ineffective photoresist layer.
One approach to addressing the foregoing drawback is to calibrate the apparatus 10. Calibration can both improve the uniformity with which a given apparatus dispenses the photoresist, and improve the consistency of results obtained from one apparatus to the next. One approach to performing the calibration is to open and close the dispense valve 131 over a period of 0.1 second while monitoring the dispense nozzle 43 by eye, and reducing the rate at which the dispense valve 131 opens if more than one drop of photoresist exits the dispense nozzle 43. The process is repeated until only a single drop exits the dispense nozzle 43. The resulting rate at which the dispense valve 131 opens is then used when dispensing the full amount of photoresist on the surface of the wafer 12.
One drawback with the foregoing approach is that it is typically not repeatable. For example, different calibration runs can produce single drops having different sizes, and the drop size can vary from one apparatus to the next. Accordingly, the existing methods for calibrating the apparatus 10 are not sufficiently accurate because they can produce photoresist layers having thicknesses that vary by up to 100 angstroms depending on which apparatus dispenses the photoresist.
The present invention is directed toward methods for dispensing a flowable substance on a microelectronic substrate. In one aspect of the invention, the method can include dispensing a portion of the flowable substance on a surface of the microelectronic substrate and receiving an image of at least some of the flowable substance on the surface of the microelectronic substrate. The method can further include comparing a characteristic of the image with a pre-selected characteristic, or comparing a time required to dispense the flowable substance with a pre-selected time by reference to the image, or both comparing the image and the time. The method can still further include adjusting a characteristic of the dispense process when the image differs from the pre-selected image by a least a predetermined amount, or when the time differs from the pre-selected time by at least a predetermined amount, or both.
In another aspect of the invention, the method can further include selecting the flowable substance to include a photoresist material. Comparing the image or the time and adjusting a characteristic of the dispense process can be performed by digital computer. Adjusting a characteristic of the dispense process can include adjusting a rate at which a valve, positioned along a flow path of the flowable substance, changes from a closed state to an open state.
In still a further aspect of the invention, the method can include receiving an image of a field that includes at least some of the flowable substance on the surface of the microelectronic substrate. Based on the image, the method can further include determining an elapsed time between a first point in time and a second point in time, the second point in time corresponding to a point at which a selected fraction of the field is at least approximately covered with the flowable substance. The method can further include determining an error value between the elapsed time and a target elapsed time and adjusting a characteristic of a manner in which the flowable substance is directed toward the microelectronic substrate when the error value exceeds a target error value by a pre-selected amount. For example, the method can, include determining an elapsed time from when a dispense valve is directed to change from a closed state to an open state, to a time at which about 20% of the image field is covered with the flowable substance. The target time can be about 0.4 seconds, and the target error value for the time can be about 0.01 second.